The purpose of this study is to determine the safety of hydroxyurea in children with sickle cell anemia between the ages 5 and 15 years of age, by giving an escalating dose regimen of the drug and monitoring hematologic, renal and hepatic indicies, and monitoring the subjects' growth. The study will also provide information on the effects of hydroxyurea on the level of fetal hemoglobin, in the percentage of F-cells, the hemoglobin and MCV in children with sickle cell anemia.